Ch. 15
COORDINATION AND
RESPONSE IN PLANTS
DR HUMERA ZEB
Tropisms
 Tropisms are growth movements related to directional
stimuli, for example, a shoot will grow towards a source of
light but away from the direction of gravity.
 Growth movements of this kind are usually in response to
the direction of light or gravity.
Responses to light are called phototropisms; responses to
gravity are gravitropisms.
 If the plant organ responds by growing
towards the stimulus, the response is
said to be ‘positive’.
 If the response is growth away from the
stimulus it is said to be ‘negative’.
 For example, if a plant is placed
horizontally, its stem will change its
direction and grow upwards, away from
gravity
Safety:Eye protection must be worn.
Experiments on tropisms
1 Gravitropism in pea radicles
• Ideally this experiment uses a piece of apparatus called a clinostat. A clinostat is a
clockwork or electric turntable, which can be set to rotate slowly about four times an
hour.
• Soak about 20 peas in water for a day and then let them germinate in a vertical roll of
moist blotting paper.
• After 3 days, choose 12 seedlings with straight radicles and pin six of these to the
turntable of a clinostat so that the radicles are horizontal. Although gravity is pulling
sideways on their roots, it will pull equally on all sides as they rotate.
• Pin another six seedlings to a cork that will fit in a wide-mouthed jar. Leave the jar on its
side.
• Place the jar and the clinostat in the same conditions of lighting or leave them in
darkness for 2 days.
Result
The radicles in the clinostat will continue to grow
horizontally, but those in the jar will have changed
their direction of growth to grow vertically downwards
Interpretation
 The stationary radicles have responded to the stimulus of one-sided gravity by growing
towards it. The radicles are positively gravitropic.
 The radicles in the clinostat are the controls. Rotation of the clinostat has allowed gravity to
act on all sides equally and there is no one-sided stimulus, even though the radicles
were horizontal.
Safety:Eye protection must be worn.
Experiments on tropisms
2 Phototropism in shoots
 Select two potted seedlings (e.g. sunflower or runner bean) of similar
size and water them both.
 Place one of them under a cardboard box with a window cut in one
side so that light reaches the shoot from only one direction
 Place the other plant in an identical situation but on a clinostat. This
will rotate the plant about four times per hour and expose each side of
the shoot equally to the source of light. This is the control.
Result
After 1 or 2 days, the two plants are removed from the boxes
and compared. It will be found that the stem of the plant
with one-sided illumination has changed its direction of
growth and is growing towards the light .
The control shoot has continued to grow vertically.
Interpretation
 The results suggest that the young shoot has responded to onesided lighting by growing towards the light. The shoot is said
to be positively phototropic because it grows towards the
direction of the stimulus.
 However, the results of an experiment with a single plant
cannot be used to draw conclusions that apply to all green
plants. The experiment described here is more of an
illustration than a critical investigation. To investigate
phototropisms thoroughly, many plants from a wide variety of
species would have to be used.
Advantages of tropic responses
Positive phototropism of shoots
 By growing towards the source of light, a shoot brings its leaves into the best
situation for photosynthesis.
 Similarly, the flowers are brought into an exposed position where they are most
likely to be seen and pollinated by flying insects.
Negative gravitropism in shoots
 Shoots that are negatively gravitropic grow vertically. This lifts the leaves and
flowers above the ground and helps the plant to compete for light and carbon
dioxide.
 The flowers are brought into a beneficial position for insect or wind pollination.
 Seed dispersal may be more effective from fruits on a long, vertical stem. However,
these advantages are a product of a tall shoot rather than negative gravitropism.
 Stems that form rhizomes (stems that grow underground) are not negatively
gravitropic; they grow horizontally below the ground, though the shoots that grow
up from them are negatively gravitropic.
Positive gravitropism in roots
By growing towards gravity, roots move deeper into the soil, which is their means of
anchorage and their source of water and mineral ions.
Plant growth substances and tropisms
Control of growth
 In animals and plants, the growth rate and amount of growth
are controlled by chemicals: hormones in animals and growth
substances in plants.
 One of the growth substances is auxin. It is produced in the
tips of actively growing roots and shoots and moves by
diffusion to the regions of extension where it stimulates cell
enlargement
Summary of control of shoot growth by auxin
 In the case of phototropism, scientists accept that the distribution of growth
substance causes reduced extension on the illuminated side and/or increased
extension on the non-illuminated side.
 When a shoot is exposed to light from one side, auxins that have been produced
by the tip move towards the shaded side of the shoot (or the auxins are
destroyed on the light side), causing an unequal distribution.
 Cells on the shaded side are stimulated to elongate more than those on the
light side.
 The unequal growth causes the stem to elongate and bend towards the light.
Growth of a shoot towards light is called positive phototropism.
 If a shoot is placed horizontally in the absence of light, auxins build up on the
lower side of the shoot, due to gravity.
 This makes the cells on the lower side elongate faster than those on the upper
side, so the shoot bends upwards.
 This is called negative gravitropism.
Classic experiments to test how
auxins work
 Wheat and other grass species belong to the
monocotyledon group of flowering plants.
 When wheat seeds germinate (start to
grow) they produce a shoot covered by a
protective sheath called a coleoptile.
 This helps to prevent damage to the new
leaves as they push through the soil.
 The coleoptile shows responses to light and
gravity in a similar way to other plant
parts.
 Wheat coleoptiles only take 2 or 3 days to
grow and they show responses very quickly,
so they are ideal for tropism experiments.
 The tip of the coleoptile, where it is
expected that auxins would be produced,
can be cut off without killing the plant,
effectively removing the source of
the auxin.
Results
A No elongation of the coleoptile occurs and there is no bending.
B The coleoptile grows taller and bends towards the light.
C The coleoptile grows taller, but there is no bending.
D The coleoptile grows taller and bends towards the light.
Interpretation
In A, the source of auxin has been removed. Auxin is needed to stimulate elongation and stimulates
a response to light. It could also be argued that the tip provides cells for growth and this source of
cells has been removed.
In B, auxin is produced by the tip of the coleoptile. It diffuses down the coleoptile and collects on
the shaded side (or is destroyed by the light on the light side). Cells on the shaded side respond to
the auxin by elongating faster than on the light side, causing the coleoptile to grow towards the
light.
In C, auxin is produced by the tip and diffuses down, causing all cells on both sides of the coleoptile
to elongate at an equal rate, and so causing an increase in length. However, the black paper
prevents the light influencing the auxin, so there is no response to the direction of light.
In D, auxin is produced by the tip of the coleoptile. It diffuses into the agar block. When the agar
block is replaced on the cut coleoptile, the auxin diffuses down from the agar and collects on the
shaded side of the coleoptile (or is destroyed by the light on the light side). Cells on the shaded side
respond to the auxin by elongating faster than on the light side, causing the coleoptile to grow
towards the light.